1. Field of the Invention
The invention pertains to a mobile crane with an undercarriage and with a superstructure mounted rotatably on the undercarriage, which superstructure has a main boom consisting of a base box unit and several telescoping sections, up to a maximum predetermined number of which are guided inside each other and can be extended telescopically from the base box unit. The main boom is supported on the superstructure so that it can rotate around a horizontal axis, and its rake can be adjusted by means of a hydraulic luffing ram hinged to the superstructure and to the base box. For a mobile crane of this type, the axle loads which the crane can accept as it is traveling over the highway represent a limit on the total allowable weight of the main boom. A lifting capacity increasing arrangement provided at least temporarily on the main boom can increase the lifting capacity of the fully extended main boom significantly beyond that which would be the case without the lifting capacity increasing arrangement.
2. Description of the Related Art
A crane of this type is known from, for example, DE 31 13 763 A1, which corresponds to GB 2,096,097. The lifting capacity increasing arrangement described here consists of a cable guying system with two cables, which are nearly parallel to the load plane, that is, to the plane defined by the hoist cable and the longitudinal axis of the main boom. The cables are attached to a point at the upper end of the extended main boom and are trained over a guy support, which extends upward at approximately a right angle to the longitudinal axis of the main boom. The other end of the guying is attached to the base of the base box of the main boom. This guying brings about a considerable increase in the stiffness and buckling strength of the extended main boom in the load plane. As a result, the lifting capacity of the main boom is also considerably increased.
A similar lifting capacity increasing arrangement in the form of cable guying is known from DE 198 02 187 A1. Here, one end of the hauling cable used for the guying is permanently connected to a boom extension, which is attached to the head of the extended main boom. The other end of the hauling cable is again wound up on a mechanically actuated cable drum, which is attached to a guy support connected to the base box of the telescoping boom. The hauling cable is trained over a return sheave at the tip of the guy support and proceeds to the cable drum. The tip of the guy support itself is connected to the base of the base box of the telescoping boom by a fixed length of guying. Thus the overall guying system consists of a fixed length part and a variable length part. As a result, the length of the guying can be easily adjusted to the length to which the telescoping boom has been extended in the case at hand.
Another lifting capacity increasing arrangement in the form of cable guying is known from EP 1 065 166 A2. The special feature here is that the guy support is formed by two separate mast-like struts, which are both set up essentially at right angles to the longitudinal axis of the telescoping boom, but spread away from each other; that is, they enclose an angle between them which is in the range of, for example, 90°. When the boom is in a horizontal position, the two struts of the guy support therefore do not point vertically upward but rather slant in an upward direction. Guying cables, which are attached to the upper end of the telescoping boom, are trained over return sheaves, one of which is on each of the two struts of the guy support, and proceed from there to cable drums. It is obvious that the cable drums could also be mounted at the tops of the actual struts, which would make it possible to eliminate the return sheaves. The upper ends of the struts are connected in turn by way of guying of fixed length to the base area of the telescoping boom. Because the two struts of the guy support are separated, the two guying cables are at a corresponding angle to each other outside the load plane. In this way, it is possible to stabilize the telescoping boom in two planes, namely, within the load plane and also transversely to the load plane. In contrast to the previously cited lifting capacity increasing arrangements, therefore, the lateral area of the telescoping boom is also stabilized.
The guy support with cable drums and cabling requires additional space on the telescoping boom; the amount of space required is therefore significantly reduced by designing the guy support so that it can be folded down. This additional arrangement, however, also leads to a considerable increase in overall weight. Because the weight of the boom cannot exceed the axle load limits that the mobile crane can accept when the crane is traveling over the road, a mobile crane with a telescoping boom cannot usually travel by road when it is equipped with a premounted lifting capacity increasing arrangement in the form of cable guying. The lifting capacity increasing arrangement must usually be transported separately and then mounted on the crane at the construction site when the crane is needed. As a way of avoiding this problem, it is sometimes possible to compensate for the increase in weight caused by the lifting capacity increasing arrangement by temporarily removing one or more of the telescoping sections from the boom. The disadvantage of this, however, is that the maximum length of the boom then available is reduced to a corresponding extent.
The ultimate goal is to have a telescoping-boom mobile crane which has a boom of the greatest possible length but which also has the greatest possible lifting capacity, that is, which can lift the greatest possible load. An increase in the working load capacity can be obtained in principle by a more solid design of the profiles used for the individual telescoping sections. This, however, would lead in turn to a corresponding increase in weight, which is not usually permitted because of the axle load limitations.